1. Field of the Invention
The present invention relates to a top-emission type organic electroluminescence display device formed by forming a reflection layer between double passivation layers on an upper portion of a thin film transistor, thereby directly coupling one electrode of source/drain electrodes with a transparent electrode layer that is a first electrode layer.
2. Description of Related Art
Generally, an organic electroluminescence display device in flat panel display is noticed as a next generation flat panel display in the future since it has merits of wide use temperature range, strong resistance against impact or vibration, wide viewing angle and clean motion picture due to fast response speed compared with other flat panel displays.
The organic electroluminescence display device uses a phenomenon that light is generated through a process in which the excited state is dropped to the ground state that is a stabilized state again after electrons and holes form electron-hole pairs, or carriers are excited to a higher energy state.
The organic electroluminescence display device is divided into a bottom-emission type organic electroluminescence display device in which the light generated by the phenomenon is emitted to a lower side of substrate and a top-emission type organic electroluminescence display device in which the light is emitted to an upper side of the substrate according to position of a reflection layer. Furthermore, the organic electroluminescence display device is divided into a passive matrix type organic electroluminescence display device and an active matrix type organic electroluminescence display device according to driving method of the organic electroluminescence display device, wherein the passive matrix type organic electroluminescence display device is driven in a line by line scanning method as organic light emitting devices are formed on a part where bus lines of the anode and bus lines of the cathode cross each other, and the active matrix type organic electroluminescence display device is driven by controlling on/off per each organic light emitting device as one or more of thin film transistors are formed per one organic light emitting device.
Concretely, the active matrix type organic electroluminescence display device comprises a plurality of gate lines, a plurality of data lines, a plurality of power supply lines and a plurality of pixels.
FIG. 1 is a plane figure for showing a unit pixel of an active matrix type organic electroluminescence display device.
Referring to FIG. 1, one pixel consists of two thin film transistors and one capacitor comprising a switching thin film transistor TS coupled with a corresponding gate line 110 in a plurality of gate lines and a corresponding data line 120 in a plurality of data lines, a driving thin film transistor TD for driving organic electroluminescence devices P coupled with power supply lines 130, and a capacitor C for producing a current source of the driving thin film transistor TD.
FIG. 2 illustrates a cross sectional structure taken along a line I-I′ of FIG. 1.
Referring to FIG. 2, a thin film transistor comprising semiconductor layer 12, gate electrode 15, source/drain regions 13-1, 13-2 and source/drain electrodes 18-1, 18-2 is formed on an insulating substrate 10 by a certain semiconductor process in an organic electroluminescence display device.
Double passivation layers 19 are formed by forming an inorganic insulating layer 19-1 for insulation and an organic planarization layer 19-2 for planarizing devices on a layer of the source/drain electrodes 18-1, 18-2 above a substrate 10 on which the thin film transistor is formed. The organic planarization layer 19-2 is formed by laying up acryl based polymer or BCB (benzocyclobutene) that is an insulating material and capable of being planarized as the organic planarization layer 19-2 on an upper part of the inorganic insulating layer 19-1 after forming the inorganic insulating layer 19-1 by laying up an inorganic insulating material such as silicon nitride (SiNx) and silicon oxide (SiOx) as an inorganic insulating layer on the substrate 10.
Next, a contact hole 20 for exposing one of the source/drain electrodes 18-1, 18-2 to the outside is formed by etching a certain part of the double passivation layers 19 consisted of the inorganic insulating layer 19-1 and the organic planarization layer 19-2.
Subsequently, a reflection electrode 21 is formed by patterning the reflection electrode material after laying up a reflection electrode material on the contact hole 20 and the double passivation layers 19 on the substrate, and a top-emission type organic electroluminescence display device is formed by organic layer 23 and second electrode layer 24 on an upper part of the reflection electrode 21.
The first electrode layer 21 is formed by adopting a reflection electrode having superior reflection characteristics, and a conductive material having reflection characteristics as well as proper work function is used as the reflection electrode. However, the first electrode layer is generally fabricated in a multilayer structure in which a reflection layer 21-1 having superior reflection efficiency is separately formed, and a transparent electrode layer 21-2 having other conductivity is formed on an upper part of the reflection layer 21-1 since a proper single material simultaneously satisfying the characteristics does not exist up to now.
As known in public, single metals including silver (Ag), aluminum (Al), chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti) and tantalum (Ta) and alloy of the single metals are used as a material composing the reflection layer 21-1, and ITO (indium tin oxide) or IZO (indium zinc oxide) is widely used as a material composing the transparent electrode layer 21-2. Aluminum or aluminum alloy and ITO are most widely used up to now considering reflection efficiency and work function. Galvanic corrosion phenomenon is generated on an interface between dissimilar metals if a multilayer structure is adopted as a reflection electrode that is the first electrode layer 21, and a metal oxide film layer such as Al2O3 is easily formed particularly when a metal used as the reflection layer 21-1, for example, aluminum is exposed to the air. As a result of that, a contact resistance on an interface between the electrodes shows a very unstable distribution as the galvanic corrosion phenomenon between the reflection layer 21-1 and the transparent electrode layer 21-2 is diffused along the interface between the layers, and the contact resistance between the electrodes is radically increased by the formed metal oxide film layer.
According to FIG. 2, the reflection layer 21-1 is constructed in such a structure that the reflection layer 21-1 is electrically coupled with one of the source/drain electrodes 18-1, 18-2 of the thin film transistor, that is, the drain electrode 18-2. As current impressed to drive an organic electroluminescence display device is transferred to the reflection electrode 21 by passing through the contact hole 20 via the drain electrode 18-2 in the above structure, the unstable contact resistance between the reflection layer 21-1 and the transparent electrode layer 21-2 greatly lowers quality of the picture materialized by generating a luminance non-uniformity phenomenon in which some colors between pixels are brightly materialized while other colors are blackly materialized during driving of a top-emission type organic electroluminescence display device. Additionally, there are problems that luminance of pixels is lowered as the light not proceeded to the front surface of the substrate 10, but it is lost to the rear side as illustrated in FIG. 2 after a light generated in an emitting layer of the organic layer 23 is reflected from the reflection film 21-1.